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Insulating a Walkout Basement

Josh_Jones | Posted in Green Building Techniques on

Hi all – long time lurker, first time question asker. I live near Seattle (Zone 4 Marine) and I am taking seismic retrofits & other remodeling as an opportunity to help with energy consumption.

The main level is basically a walk out basement. One corner is completely below grade, the opposite diagonal corner is completely above grade. My question is since the foundation wall level stair-steps around the perimeter of the house, I’m left with either following the contour of the wall (ugly) or building another interior wall that allows the inside surface to be flat (the route we are going). For below grade, I’m doing R-5/R-6 against the concrete and a 2×4 wall up against the foamboard insulation. But since the wall is slab to ceiling, once it is above the foundation wall, I’m basically left with a double wall situation. Given our moist climate, I’m pretty cautious about things that could lead to mold. Overall, the drainage appears to work, I’ve yet to find moisture ingress issues (we have cantilevers and generous overhangs, too). We recently installed a heat pump, so summer time de-humidification is generally under control (I’ve been monitoring humidity in several spots with sensors that log their data). I’m also considering adding a de-humidifier to provide some suspenders to my belt.

Anyways – my thought is leave the exterior wall cavity empty but vapor breathable (plywood & tyvek) with a rigid foam air control layer on the inside of the outside wall. Then fibrous (rockwool) insulation filling the interior side. The idea is block airflow between the two walls and leave them to dry to their respective side.

My retrofit for upper story (completely above grade) will be 1″ foam & R-15 roxul between the studs (since I’m doing seismic and we have sh*tty thermoply cardboard sheathing, I’m stripping siding and putting plywood sheathing on – and thus doing this from the outside). I’m doing rainscreen all around, too.

My question is does this seem at all problematic? My other question is – would there be any benefit to adding any cavity insulation on the exterior wall – mostly regarding moisture control – as I’ll already have nearly R-40 with R-25 of that being with practically no thermal bridging or should i just leave it empty?

Thanks in advance!

Josh

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Replies

  1. DCContrarian | | #1

    The simplest is to have the same construction for the entire height of the interior basement wall, and foam against the foundation and then a stud wall filled with rockwool is a relatively safe construction. It's also safe above ground. I see no reason not to put unfaced insulation in the exterior cavity of the above-ground portion, it's drying to the exterior and it's a cheap and easy opportunity to add more insulation, and more is always good.

    1. Josh_Jones | | #2

      Thanks for the reply! The main reason I'm choosing to follow the contour of the existing wall (both the wood and concrete portions) with the foam (my air barrier) is due to buildability. It's easier to install the foam on the existing wall then build the new wall. Easier to detail my foam that way. And yeah, I plan on all unfaced insulation (kind of a given since I'm using Rockwool).

  2. Expert Member
    Malcolm Taylor | | #3

    Josh,

    I'd suggest 0ne of the following:.

    - You can stop the interior walls at the top of the foundation, leaving a ledge. The part against the concrete wall gets built like any basement (foam, stud-wall). The part above you would treat the same way you intend to insulate all the rest of the stud-walls in the house.

    - Run the interior wall right up as you show in your section. The part against the concrete gets the same treatment, but the part above I would design like most other double-walls, and just use batts to fill the thickened cavities. I don't see much point in extending the foam right up. You will end up with these walls proportionately very over-insulated compared to everything else in the house, and the foam (vapour-retarder) is close en0ugh to the exterior that it might cause moisture problems.

    - Do exactly as you show in the section, and leave the outer bays empty, making sure the foam is thick enough to avoid condensation.

    1. Josh_Jones | | #4

      Malcolm,

      Thanks for the response!

      First option: we had the ledge before - it was ugly - especially since it's not at the same level all the way around. Since that means a thicker wall cavity on the top, I thought I'd use the opportunity to add more insulation.

      Second option: yeah, my ambivalence about leaving the outer (structural) wall empty is completely based on moisture and trying to keep the dew/condensation point within an insulation layer that water can't condense on (and blocking the flow of air to keep indoor air indoors and outdoor air outdoors).

      Third option is the one that I'm leaning towards. With the depth of the space, I should be able to do 2" of foam along with 7" of cavity (inside cavity only, not outdoor cavity). That should be a stackup of R-10 (min) foam and R-30 Rockwool which means my foam accounts for 25% of the R-value which is greater than the 16% needed for Marine 4 (according to Martin's articles on exterior foam).

      I know it will be kinda "top heavy" insulation wise. I'm doing what I can where I can since this is a renovation. Given it is already going to have a disproportionate amount of insulation, I'm wanting to favor buildability and least moisture risk over getting all the R-value I can. I wanted to make sure that my thought that leaving the exterior cavity empty and protected from indoor air by the foam insulation wasn't insane.

      1. Expert Member
        Malcolm Taylor | | #5

        Josh,

        No. At first glance it seems like a missed opportunity leaving the bays empty, but as you say - there is going to be more than enough insulation there, and the empty bays will have great drying potential.

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